Calorific value refers to the amount of energy released by the complete combustion of a material, generally expressed in MJ/kg or kWh/kg. This quantity is essential for evaluating the energy efficiency of fuels and guiding industrial choices towards the most efficient solutions. Used in the energy, materials, and waste recovery sectors, calorific value analysis allows for objective comparisons between different energy sources. In this context, specific heat analysis provides valuable complementary data to better understand the thermal behavior of materials before or during combustion. Advanced techniques such as differential scanning calorimetry (DSC) allow for the precise measurement of these thermal properties in the laboratory. This article explores in detail the key concepts related to calorific value, its types, units, and associated analytical methods.
1. What is calorific value?
Scientific definition
Calorific value, also known as heat of combustion, is defined as the amount of heat released by the complete combustion of a unit of fuel in the presence of oxygen. This combustion produces mainly carbon dioxide (CO₂), water (H₂O), and heat, which can then be recovered for energy purposes.
In the international system, calorific value is expressed in joules per kilogram (J/kg), but it can also be indicated in kilowatt-hours per cubic metre (kWh/Nm³) for gases or in kilojoules per mole (kJ/mol) for precise chemical calculations. These different units make it possible to adapt to the nature of the fuels and to varied industrial contexts.
Importance of calorific value
Calorific value plays a crucial role in energy choices. It is used for:
- Compare fuel efficiency : For example, determining whether wood, coal, or methane is best suited for a specific application.
- Evaluate equipment performance : The efficiency of boilers or engines depends directly on the calorific value of the fuels used.
- Optimizing energy management : Industries choose their energy sources based on calorific value to maximize their efficiency and reduce their costs.
Link with combustion enthalpy
From a thermodynamic point of view, calorific value is the opposite of the enthalpy of combustion (ΔcH0)(Δ c H 0), which measures the change in total energy during a chemical reaction. A negative enthalpy indicates a release of energy, characteristic of exothermic reactions such as combustion.
Adapted units and formats
Units of measurement vary depending on needs and sectors:
- Solids : expressed in joules or kilojoules per kilogram (J/kg, kJ/kg).
- Liquids and gases : expressed in kilojoules per litre (kJ/L) or kilowatt-hours per cubic metre (kWh/Nm³).
- Moles : Useful in chemical analyses for precise calculations.
By adapting units to the specific properties of fuels, it is possible to ensure clear and consistent understanding in various contexts, from industrial production to scientific research.
The two types of calorific value
Calorific value falls into two main categories, each with specific applications.
- Gross heating value (GHV) : GHV includes the latent heat of condensation of water produced during combustion. This value represents the total energy released, including heat recoverable through condensation. It is a comprehensive measurement, often used in systems such as condensing boilers, which exploit this additional energy.
- Net heating value (NCV) : The NCV excludes the latent heat of vaporization of water. It therefore represents only the sensible heat recoverable when the water vapor remains in the gaseous state. The NCV is generally used in systems where condensed water is not recovered, such as heat engines or conventional boilers.
These two types allow the use of calorific value to be modulated according to the available technologies and the desired energy objectives. calorific value analysis (PCI PCS) allows these values to be precisely determined to guide technical choices based on the context of use. PCS is, for example, particularly relevant for modern installations aiming to maximize their energy efficiency.
2. How to measure calorific value?
Laboratory measurement methods
The measurement of calorific value, particularly the higher calorific value (HCV), is carried out using a combustion calorimeter, also called a bomb calorimeter . This technique consists of evaluating the quantity of heat released by the complete combustion of a sample in a controlled environment.
- Principle of the bomb calorimeter :
- The sample, weighed precisely, is placed in a sealed combustion chamber (the bomb calorimeter).
- The bomb is filled with pure oxygen under pressure to ensure complete combustion.
- It is immersed in a quantity of water whose temperature is continuously monitored.
- Combustion is initiated by a spark, and the heat released by the reaction is transferred to the surrounding water.
- By measuring the rise in water temperature, the total amount of energy released is calculated.
- Steps of the process :
- Sample preparation: weighing, placement in the bomb.
- Oxygen filling for optimal combustion.
- Recording of temperature variations.
- Data analysis to calculate the PCS.
Example of devices used
Devices like the IKA C200 are particularly suitable for low-volume or educational analyses. This type of calorimeter is compact, economical, and easy to use, making it ideal for laboratories performing a limited number of measurements or for educational applications.
For industrial analyses or high-precision studies, more advanced and automated models can be used, allowing for increased reproducibility and efficiency. These devices often incorporate software that facilitates data analysis.
Importance of standards and precision
The measurement of calorific value must comply with international standards such as ISO 18125 , which specifies methods for determining the PCS for solid biofuels. This standard guarantees:
- Optimal measurement accuracy.
- Comparability of results between different laboratories.
- The use of certified reference materials, such as benzoic acid, to calibrate devices.
Analyses carried out in ISO 17025 accredited laboratories provide an additional guarantee of quality and reliability, which is essential in demanding fields such as the energy industry.
Advantages and limitations of current methods
Calorimetric methods offer high accuracy, but require careful sample preparation and strict measurement conditions. The presence of water or contaminants in the samples can alter the results, requiring corrections or additional analyses to determine the PCI from the PCS.
3. How to calculate PCI and PCS?
Defining key parameters
To perform the calculations, it is necessary to understand the following:
- Higher calorific value (HCV) : It represents the total energy released during the complete combustion of a fuel, including the latent heat of condensation of water.
- Lower calorific value (LCV) : This is the useful energy released excluding the latent heat linked to the water vapor formed during combustion.
These two quantities are linked by the following formula:
Or :
- Hv Hv is the latent heat of vaporization of water (about 2442 kJ/kg at 25 °C).
- nH2O nH 2 O is the amount of water formed per kilogram of fuel.
Calculation steps
Determine the PCS :
- PCS is measured directly using a combustion calorimeter, as explained previously.
- Results are expressed in kilojoules per kilogram (kJ/kg) or kilojoules per mole (kJ/mol)
Calculate the amount of water formed :
It depends on the chemical composition of the fuel, in particular its hydrogen ( H ) content.
When a hydrocarbon burns, each hydrogen atom contributes to the formation of a water molecule ( H2O ) :
Apply the correction to obtain the PCI
- Multiply the amount of water formed by the latent heat of vaporization ( Hv ).
- Subtract this value from the PCS to obtain the PCI.
Practical example: Methane
Methane (CH4 CH 4) is a fuel widely used in industries and homes. Typical values are:
- PCS : 55.5 MJ/kg
- Composition : 25% hydrogen by mass.
The amount of water formed when one kilogram of methane is burned is calculated as follows:
nH2O=0.25 kg×9=2.25 kg of water
Applying the PCI formula:
PCI=55.5 MJ/kg−(2.25 kg×2.442 MJ/kg)
PCI=55.5 MJ/kg−5.49 MJ/kg=50.01 MJ/kg
The PCI of methane is therefore approximately 50 MJ/kg , which corresponds to the energy actually available if the water vapor is not condensed.
Importance of calculations for applications
- Condensing boilers : These devices exploit the difference between PCS and PCI by recovering latent heat, thus maximizing their energy efficiency.
- Petroleum and chemical industries : PCI and PCS calculations allow for standardized comparisons between fuels.
- Regulations : Calculations are essential to meet energy and environmental standards.
4. What is the PCI of methane?
Energy values of methane
Methane (CH4 CH 4) is a simple hydrocarbon composed of one carbon atom and four hydrogen atoms. Its combustion releases a significant amount of energy, making it a valuable energy resource. Typical values associated with methane are:
- Higher calorific value (HCV) : 55.5 MJ/kg
- Lower calorific value (LCV) : Approximately 50 MJ/kg
The difference between these two values lies in the consideration of the latent heat of condensation of water produced during combustion. By excluding this latent energy, the PCI represents the energy actually usable in systems where the condensation of water vapor is not exploited.
Combustion and water formation
The combustion of methane follows the following chemical reaction:
CH4+2O2→CO2+2H2O+Energy
For every methane molecule burned, two water molecules are produced. These water molecules, in the form of vapor, contain some of the energy released, which is why the LHV is lower than the HHV.
Applications of PCI of methane
- Domestic and industrial boilers :
- Conventional boilers use the PCI as a reference, because they do not recover the latent energy from the water vapor.
- Condensing boilers exploit the difference between PCI and PCS to improve their energy efficiency, which can reach 110% when expressed in relation to PCI.
- Electricity production :
- Natural gas-fired power plants use methane to generate electricity. Accurate knowledge of the LHV helps optimize the efficiency of gas turbines.
- Transport and distribution :
- PCI is a key parameter for assessing the energy efficiency of compressed (CNG) or liquefied (LNG) methane in transportation systems.
- Chemical industries :
- Methane serves as a raw material for the production of hydrogen, an important energy carrier. The LHV of methane directly influences the efficiency of reforming processes.
Environmental impact and efficiency
The LHV of methane is an important indicator for assessing its environmental impact. Although it is a fossil fuel, methane emits less carbon dioxide per unit of energy produced than coal or oil. However, methane leaks into the atmosphere can reduce these benefits, as methane is a potent greenhouse gas.
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5. How to measure heat capacity?
Definition and key concepts
Heat capacity, also called heat capacity or specific heat , is defined as the amount of heat required to raise the temperature of one gram of a material by one degree Celsius (°C). It is expressed in joules per gram per degree Celsius (J/g/°C) .
Two variants are generally distinguished:
- Specific heat capacity : Measured per unit mass, it is often used for solids and liquids.
- Molar heat capacity : Expressed per mole of substance (J/mol/°C), it is used in chemistry and thermodynamics.
Heat capacity depends on several factors, such as the chemical composition of the sample, its physical state, and the temperature at which the measurement is made.
Measurement methods
calorimetry techniques using precise instruments to monitor heat exchanges.
Differential scanning calorimetry (DSC) :
- The DSC method is commonly used to measure the heat capacity of solids and liquids.
- It involves heating the sample and an inert reference at a constant rate, while recording the difference in heat flow between the two.
Microcalorimetry :
- Microcalorimetry is used for small samples or for measurements requiring extreme precision.
- It is particularly suitable for sensitive or heterogeneous materials.
Specific calorimeters :
- Some calorimeters allow measurements in extended temperature ranges, from 80°C to 600°C , or even beyond with advanced devices such as ATD coupled with DSC.
Experimental conditions
To ensure reliable measurements, several precautions must be taken:
- Thermal stability : The sample must not undergo phase transition, degradation, or evaporation during measurement.
- Sample homogeneity : Thermal properties should be uniform throughout the sample.
- Sample Amount : Typically, between a few milligrams and a few grams are needed for an accurate measurement.
Applications of heat capacity
- Materials design :
- Heat capacity helps assess the thermal stability of materials, a key factor for industrial applications.
- It allows the properties of thermal insulators or materials used in extreme environments to be optimized.
- Energy optimization :
- Heat capacity data is used to model heat exchanges in industrial systems, thereby reducing energy losses.
- They are also essential in combustion analyses and chemical reforming processes.
- Scientific research :
- In chemistry, molar heat capacity is used to calculate thermodynamic parameters such as enthalpy and entropy.
- In the food industry, it helps assess the thermal stability of foods during processing.
6. Examples of calorific value values
Calorific value of fossil fuels
Fossil fuels such as coal, natural gas, and oil are primary energy sources widely used in industry and for electricity generation. Here are some typical values:
- Coal :
- PCS: 25 to 35 MJ/kg depending on the type of coal (lignite, anthracite).
- Used mainly for electricity production in thermal power plants.
- Crude oil :
- PCS: 42 to 45 MJ/kg.
- Serves as a base for fuels such as gasoline and diesel.
- Natural gas (methane) :
- PCS: 55.5 MJ/kg.
- Mainly used in domestic boilers and power plants.
Calorific value of biofuels
Biofuels, derived from organic matter, represent a more sustainable alternative to fossil fuels. Their calorific value varies depending on moisture content and chemical composition.
- Drink :
- PCS: 15 to 20 MJ/kg for dry wood.
- Widely used in residential heating systems.
- Biomass pellets :
- PCS: 16 to 19 MJ/kg.
- Offer high energy density and are suitable for modern boilers.
- Biodiesel :
- PCS: Approximately 38 MJ/kg.
- Used in adapted engines as an alternative to diesel.
Calorific value of liquid fuels
Liquid fuels, thanks to their high energy density, are essential for transport and industry.
- Essence :
- PCS: 47.3 MJ/kg.
- Mainly used for internal combustion engines.
- Diesel :
- PCS: 44.9 MJ/kg.
- Preferred in diesel engines for its more efficient combustion.
Calorific value of hydrogen
Hydrogen is recognized for its exceptional calorific value, but its use remains limited due to the technical challenges associated with its production and storage.
- Dihydrogen (H₂) :
- PCS: 143 MJ/kg.
- Considered an energy vector of the future, particularly in fuel cells.
Comparison and interpretation
The differences in calorific value between these fuels directly influence their choice and use:
- Fuels with high PCS, such as hydrogen and methane, are preferred in modern technologies requiring high energy density.
- Biofuels, although generally having a lower PCS, are valued for their low carbon footprint and availability.
7. Standards and regulations
ISO 18125 standard: a reference for biofuels
ISO 18125 is an international standard specifically dedicated to the determination of the calorific value of solid biofuels. It defines methods for measuring the gross calorific value (GCV) under rigorous conditions.
- Methodology :
- The measurement is carried out at constant volume using a calorimetric bomb, with a reference temperature of 25°C.
- Certified benzoic acid is used as a reference material for calibrating devices.
- Applications :
- The standard is widely used to analyze matrices such as biomass pellets, wood chips, and other biofuels.
- It guarantees precise and reproducible results, essential for comparing different biofuels.
Other related standards
- Standard NF EN ISO 21654 :
- Concerns the determination of the energy properties of fossil and biogenic fuels.
- It is applicable to various matrices, such as coals and heavy oils.
- Environmentally specific standards :
- European standards, such as EC Regulation No. 1935/2004, regulate the impact of fuels on the environment.
- US regulations (FDA) supplement these requirements for materials used in specific contexts, such as food contact packaging.
Role of accredited laboratories
Laboratories accredited to ISO 17025 play a key role in implementing these standards. Their accreditation guarantees:
- The reliability and accuracy of the results.
- The use of calibrated equipment and validated methodologies.
- Full compliance with the requirements of international standards.
By working with accredited laboratories, industries benefit from actionable data for strategic decision-making, whether in the choice of fuels or in the validation of energy processes.
Importance of regulations for end users
Standards are not limited to laboratories; they extend to the use of fuels by industries and consumers. For example:
- Energy industries : They must comply with standards to guarantee the safety of their installations and the performance of fuels.
- Boiler manufacturers : They use standardized data to design equipment suitable for different types of fuel.
- Consumers : Labels and data sheets provided on fuels, such as wood or gas, include standardized values to enable informed comparisons.
